Electrical contact for silicon carbide members

ABSTRACT

Tantalum and gold alloy electrical contacts are applied to selected areas of a body of silicon carbide. An oxidized surface is first produced on the silicon carbide body and removed from those portions of the surface to which the alloy contact of the body is to be applied. A thin layer of aluminum is deposited on only the oxidized surface. Thin films of tantalum and gold are alternately deposited on the exposed surface of the silicon carbide. Any tantalum and gold deposited on the aluminum forms an alloy which is readily removed by an etchant which attacks tantalum-gold-aluminum intermetallics but will not attack the tantalum-gold alloy electrical contact of the body.

United States Patent Formigoni et a1.

[ 51 May 16,1972

[72] Inventors: Napolean P. Formigoni, Pittsburgh; John S. Roberts,Export, both of Pa.

Westinghouse Electric Corporation, Pittsburgh, Pa.

[22] Filed: June 20,1969

[21] Appl.No.: 835,017

[73] Assignee:

[56] References Cited UNITED STATES PATENTS 3,492,719 2/1970 Zeitman eta1 ..29/590 3,516,914 6/1970 Hall ..l56/l7 Primary Examiner.lohn F.Campbell Assistant Examiner-D. M. Heist Att0rneyF. Shapoe and C. L.Menzemer ABS'I RACT Tantalum and gold alloy electrical contacts areapplied to selected areas of a body of silicon carbide. An oxidizedsurface is first produced on the silicon carbide body and removed fromthose portions of the surface to which the alloy contact of the body isto be applied. A thin layer of aluminum is deposited on only theoxidized surface. Thin films of tantalum and gold are alternatelydeposited on the exposed surface of the silicon carbide. Any tantalumand gold deposited on the aluminum forms an alloy which is readilyremoved by an etchant which attacks tantalum-gold-aluminumintermetallics but will not attack the tantalum-gold alloy electricalcontact of i the body.

7 Claims, 2 Drawing Figures ELECTRICAL CONTACT FOR SILICON CARBIDEMEMBERS GOVERNMENT CONTRACT The invention herein described was made inthe course of, or under a contract or subcontract thereunder, with NASA.The contract is NAS 8-1 1861.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention,relates to the making of ohmic electrical contacts to bodies of siliconcarbide particularly for producing semiconductor devices, andspecifically, to an electrical contact consisting of an alloy producedby applying to the silicon carbide alternate layers of tantalum andgold, and methods for producing the same.

2. Description of the Prior Art The formation of ohmic contacts onbodies of silicon carbide semiconductor material encounters the problemsof the high temperature requirements for metallurgically alloying metalsto silicon carbide as well as the tendency of the contacts to exhibitnon-linear I-V characteristics and high contact resistance approachingan order of l megohm.

One known method of contacting silicon carbide electrically is to alloya pellet of silicon to the silicon carbide body and then affix anelectrical contact to silicon. This method is accomplished in a vacuumat a temperature of from 1,500 to l,800 C. However, this method isimpractical for small devices wherein the contact is of the order of 1mil and tolerances encountered vary from I to 1% mil. All contactspreferably should be made at the same time. Equally spaced contactsrequiring an array of pellets of silicon are impractical.

A second known method electrically contacting the silicon carbide bodyis to prepare a molten metal mixture consisting of 90 percent by weighttantalum and the balance gold. A cast is made of the molten metalmixture and preforms are prepared from the cast. These preforms are thenalloyed at a high temperature to the silicon carbide body to form anelectrical contact. However this method is of limited utility since itis suitable only for making microscopic electrical contacts to siliconcarbide and it is impractical for making electrical contacts in themicroscopic range required for microelectronic size devices whereclearances of 1 mil and tolerances between sections often are from b to1 mil.

OBJECTS OF THE INVENTION An object of this invention is to provide amethod to provide an electrical contact consisting of an alloy oftantalum and gold affixed to selected areas and in precise configurationwith close tolerances to a body of silicon carbide material.

Other objects of this invention will, in part, be obvious and will, inpart, appear hereinafter.

SUMMARY OF THE INVENTION In accordance with the teachings of thisinvention there is provided a body of silicon carbide having at leastone ohmic electrical contact consisting of a gold-tantalum alloy affixedto a selected portion of the surface of the body. The electrical contactis initially of a laminated construction before heat treatment andconsists of at least four alternate vacuum deposited thin film layers oftantalum and gold which are subsequently alloyed with each other andwith the silicon carbide of the body. The alloy contact is from about1,000 to 10,000 A in thickness. Briefly the process for applying theelectrical contact is:

1. Provide a thin silicon oxide coating on the silicon carbide body,preferably by oxidizing the body on the face perpendicular to the C-axisof a hexagonal crystal, the oxide being about 1,000 to 3,000 A thick,

2. Mask the oxide to expose selected areas where the alloy contacts areto be applied, and etch away or otherwise remove the silicon oxide fromthe exposed selected areas, and remove the mask,

3. Apply a vacuum deposited layer of aluminum on the silicon oxidecoating remaining on the silicon carbide; if the aluminum is applied toall the surfaces, it may be removed from the exposed selected areas freeof silicon oxide by appropriate masking and etching,

4. Deposit upon the surfaces of the aluminized silicon carbide body thinalternate layers of gold and tantalum, a tantalum layer being the firstapplied to the silicon carbide, at least to provide a total of fourlayers, suitable overall thickness being about 3,000 A,

5. Etching away the aluminum layer in an etchant, with or without aprior heat treatment at about 300 C., leaving only gold-tantalum layersat the selected areas, and

6. Heat the silicon carbide body at an elevated temperature to alloy thegold and tantalum layers, including metallurgical bonding of the alloyto the exposed silicon carbide to provide good ohmic contacts at theselected areas.

DRAWINGS In order to more completely understand the nature and objectsof this invention reference should be had to the drawings in which FIGS.1 and 2 are elevation views in cross-section of portion of a body ofsilicon carbide semiconductor material being processed in accordancewith the teachings of this invention.

DESCRIPTION OF THE INVENTION The electrical contact of this invention issuitable for use with any body of silicon carbide and is particularlysuitable for semiconductor devices whether or not the body has any p-njunction contained therein. In order to describe the invention moreparticularly the invention will be described relative to the afiixing ofelectrical contacts to a portion of a junctiongate type unipolartransistor.

Referring now to FIG. 1 there isshown a portion 10 of junction gate typeunipolar transistor. The portion 10 is made of hexagonal single crystalsilicon carbide semiconductor material suitably doped to form regions12, 14 and 16 of p-type semiconductivity and region 18 of n-typesemiconductivity. pn junctions 20, 22 and 24 are between the respectiveregions 12 and l8, l4 and 18, and 16 and 18. All the p-n junctions 20,22 and 24 have end portions exposed in a planar surface 26 which is alsothe top surface of the transistor. A bottom surface 28 of the transistorand the surface 26 are opposed major surfaces of the transistor.

A layer 30 of silicon oxide is formed by suitable means on at least thesurface 26 of the portion 10. The layer 30 of silicon oxide may beformed by vapor deposition on the surface 26 or by thermal conversion ofa part of the surface 26 from silicon carbide to silicon oxide. Eithermethod is suitable and is well known to those skilled in the art. Thelayer 30 must be continuous but need not be uniform in thickness. Athickness of 1,000 to 3,000 A has been found to be sufficient for thelayer 30 for the process of this invention. Employing knownphotolithographical masking techniques followed by selective etching,windows are open in the layer 30 to expose one or more selected surfaceareas of silicon carbide of regions 14, 16 and 18 to which electricalcontacts are to be made.

The layer 30 of silicon oxide enables one to form the electricalcontacts easier than if the layer 30 was not present. This layer is alsoessential to protect and isolate the surface of the silicon carbidedevice during subsequent processing. The silicon oxide of the layerconfines the tantalum and gold to the selected contact areas only duringthe alloying process step.

Other suitable methods of masking the surface 26 include, such forexample, metal masks and photoresist materials, and in some cases couldbe employed instead of oxide layer 30.

The portion 10 with the layer 30 of silicon oxide having windows thereinis disposed in a vacuum metal deposition chamber and a thin layer 32 ofaluminum is deposited over at least the entire oxide layer 30, exposedsurfaces of the regions 14, 16 and 18. The thickness of the aluminumlayer 32 is important. Sufiicient aluminum is required to form anintermetallic mixture with the gold and tantalum metals of theelectrical contact having properties which enable the selective removalof the metals from the non-contact fonning areas. If the aluminum layer32 is too thin, all of the gold-tantalum, metal is not completelystripped away from the areas where it is not desired, and if the layer32 is too thick then the gold-tantalum alloy at the electrical contactareas may be attacked by the etchants. It has been determined that wherethe gold tantalum layer has an overall thickness of 3,000 A, the layer32 of aluminum should preferably be approximately 10,000 A, and may befrom about 5,000 to 20,000 A.

The thickness of the aluminum layer 32 is small enough that the patternof the exposed contact areas etched in the layer 30 is visible throughthe layer 32. This enables accurate placement of masks byphotolithographical techniques and selective etching so that thealuminum is removed from the windows in the layer 30 of silicon oxide toagain expose the selected electrical contact areas of the regions 14, 16and 18.

The portion of the unipolar transistor is disposed in a vacuum metaldeposition chamber and alternate layers 34 and 36 of first tantalum andthen gold are deposited on at least the aluminum layer 32 and theexposed selective surface areas of the regions 14, 16 and 18 withoutbreaking the vacuum. The ratio of the total of tantalum to gold is from1:5 to 1:15 by volume. It has been determined that a minimum totalnumber of four alternate layers 34 and 36 is sufficient, but the bestresults were obtained when the total number of layers were from 8 to 10.The total thickness of the deposited gold and tantalum layers 34 and 36may range from 1,000 to 10,000 A.

Preferably, the vacuum system is equipped with facilities for sputteringgold and tantalum, for example, from separate crucibles, with movablecovers according to a suitable alternating sequence without breaking thevacuum. The silicon carbide bodies are subjected to considerable heatduring the metal deposition process. However, the amount of heat thebody is subjected to can be varied considerably without affectingappreciably the metal deposition rates.

The tantalum and gold plated portion 10 is removed from the vacuumevaporation chamber and any excess tantalum and gold over the aluminizedareas and all of the aluminum are removed by any one of several methods.One method consists of disposing the plated portion 10 in a concentratedsolution of sodium hydroxide. It appears that the sodium hydroxiderapidly attacks the aluminum, dissolving it, and the tantalum and golddeposited on the aluminum is so thin that is is readily removed byfloating it away. As mentioned before, if the layer 32 is too thick thenthe tantalum-gold layers 34 and 36 deposited on the exposed surfaces ofthe regions 14, 16 and 18 may be also attacked by the sodium hydroxidesolution.

An alternate method of removing the aluminum and any tantalum and goldfrom undesired areas when it is over aluminum is to first treat theportion 10 of the transistor with the deposited alternate layers at anelevated temperature either in a vacuum or in a controlled furnaceatmosphere wherein the atmosphere may be nitrogen, oxygen or argon.During heat treatment at about 300 C. 20 C. for up to 16 hours, itappears that the gold difluses through the intervening layers 34 oftantalum of the stratified structure and into the layer 32 of aluminumwhere an aluminum-gold-tantalum interrnetallic compound is formed. Thechemical constitution of the intermetallic mixture is such that it isreadily attacked and removed by etchants such, for example, as sodiumhydroxide and phosphoric acid while leaving the stratified contactstructure above the exposed silicon carbide surface intact.

After selective removal of the undesired metal, the remaining portionsof tantalum and gold layers 34 and 36 are disposed directly on theselected areas of silicon carbide are then subjected to an elevatedtemperature comprising an initial preheating for a period of someminutes followed by a brief flash or pulse heating for several secondsat 1,500 to l,700 C. The initial preheating is from 850 to l,050 C in avacuum, for example, from 10" torr to l0" torr. A preferred temperatureis 900 C. and a preferred vacuum is 10" torr. The initial heating of thegold and tantalum metal layers is to promote the interdiffusing ofmetals into each other without melting the gold. The heating in vacuumeliminates any complications which may arise from oxidation of any ofthe materials. This heat treatment is for a period of time sufficient toallow extensive and thorough diffusion of gold throughout the tantalumthereby obtaining a sufficient homogeneity of the metals to fonn analloy with themselves and to be alloyed to the silicon carbide. Thispreheating step was found to be necessary to avoid the segregation ofthe gold when alloying of the contact to the silicon carbide isperfonned.

It has been found that a very rapid and a high increase of temperaturefor the second heating step is desirable to obtain a good ohmiccharacteristic for the alloyed electrical contacts. Dwell times at thehigh elevated temperature of more than a few seconds are avoided sincegold may be lost by evaporation.

As specific illustration, in alloying the stratified tantalum and goldelectrical contact with itself and to the exposed silicon carbidesurfaces of the regions 14, 16 and 18, the portion 10 is heated first atemperature of from 850 to l,050 C. for a period of at least 10 minutes.A period of from 14 to 16 minutes is found to be sufficient. The heatingat the higher temperature varies with apparatus design and heating meansis performed for a pulse of time of the order of 10 seconds and notexceeding about 15 seconds at a maximum temperature of from l,500 tol,700 C. The resulting structure is shown in FIG. 2 in which electricalcontacts 38, 40, and 42 are the alloyed electrical contacts to therespective regions l8, l4 and 18.

The heating at the higher temperature for the short duration isdesirable, because it has been found that this process step improves theohmic characteristics of the tantalum-gold electrical contacts. As soonas alloying of the tantalum diffused gold with the silicon carbideoccurs, the temperature is reduced immediately to below thesolidification temperature of gold to prevent its loss by evaporation.The electrical contacts of this invention exhibit a more linear l-Vcharacteristic and have a lower contact resistance than the prior artelectrical contacts, and in particular the tantalum-gold electricalcontacts of the prior art. The electrical contacts of this inventionhave good physical adhesion to the silicon carbide material, low contactresistance, and resistance to oxidation even when the silicon carbidedevices are operated at temperatures of 500 C. Additionally, gold wireleads may be bonded to the electrical contacts of this invention by theconventional thermocompression bonding process more easily than priorart devices.-

An example illustrating the process of making an electrical contact inaccordance with the teachings of this invention is as follows:

Ohmic tantalum-gold electrical contacts were affixed to a Junction-GateUnipolar Transistor, a portion of the same design being shown in FIG. 2.Five electrical contacts were required, the contacts consisting of twosource, two gate and one drain.

After formation of the required regions in an initial n-typesemiconductivity wafer of silicon carbide, the wafer was heated to atemperature of l,l70 C. 2 20 C. for 30 minutes in the presence of argongas saturated with water vapor at l00 C. Oxidation of the wafer wascontinued for 30 minutes. The layer of silicon oxide thermally grown was2,500 A in thickness.

Employing photolithographical techniques followed by selective etchingemploying an etchant consisting of 7 parts by volume ammonium fluorideto 1 part by volume 49 percent hydrofluoric acid, windows were opened inthe thermally grown silicon oxide layer thereby exposing preselectedsurface areas of the wafer.

The wafer was then placed in a vacuum system equipped with facilitiesfor the vapor deposition of aluminum. A vacuum of torr was establishedand a layer of aluminum 10,000 A in thickness was vapor deposited on theoxide coated surface of the wafer including the windows for the contactsto the wafer. Employing photolithographical techniques and the etchant HPO the aluminum was removed from the windows in the layer of siliconoxide thereby exposing the surfaces of the silicon carbide wafer towhich the electrical contacts were to be affixed.

The wafer was then placed in a vacuum system equipped with facilitiesfor sputtering gold and tantalum according to a predetermined alternatedeposition technique. A vacuum of 10' torr was established in the systemand five layers of tantalum alternated with four layers of gold wassputtered onto at least the surface areas of the silicon wafer to whichelectrical contacts are made and the aluminum layer about each of thewindows. The proportion of gold to tantalum was 86% Au to 14% Ta byvolume (approximately 6:1) and each layer was deposited to provide itsprorata amount of metal for the overall contact. The deposition processtook 40 minutes. The overall thickness of the deposited tantalum andgold layers was 3,000 A. The wafer was then placed in a saturatedsolution of sodium hydroxide. ln 3 minutes all the aluminum and theexcess tantalum and gold had been removed from the wafer. Tantalum andgold remained in the windows of the silicon oxide layer.

The wafer was then placed in a vacuum system and a vacuum of the orderof 10 torr was established. The wafer was then heated in the vacuum to900 C. 50 C. for 15 minutes. The temperature of the wafer was thenrapidly increased to l,600 C. 100 C. and held at maximum temperature forabout 10 seconds, after which the wafer was cooled to room temperatureand removed from the vacuum system.

A visual examination of the contacts showed the contacts to be of auniform sound structure having a good surface. Electrical tests wereperformed and the contacts exhibited a near linear l-V characteristicand a low contact resistance of 10 ohms.

The wafer was sectioned and mounted for examination of some of thecontacts. All contacts exhibited a sound structure alloyed to thesilicon carbide wafer. The gold had diffused through the tantalumlayers. The silicon oxide had confined the tantalum and gold to thecontact windows during alloying. The remaining contacts were checked forphysical bonding to the wafer. The physical bond proved to be excellentwith the material of the wafer being removed with the contacts when theywere stripped from the wafer.

We claim as our invention:

1. ln theprocess of applying an ohmic electrical contact ofgold-tantalum alloy to selected areas of a silicon carbide body, thesteps comprising a. applying to the silicon carbide body, at other thanthe selected areas, a thin layer of silicon oxide and a superposed layerthereon of aluminum of a thickness of from about 5,000 to 20,000 A,

b. applying by vacuum evaporation a plurality of alternate thin layersof tantalum and gold, a tantalum layer being the initial layer incontact with exposed silicon carbide of the selected areas;

c. heating the silicon carbide body at about 300 C. for a period of timeof up to 16 hours;

d. etching the treated silicon carbide body with an etchant capable ofdissolving aluminum whereby the gold-tantalum deposited over thealuminum layer is removed; and

e. heat treating the silicon carbide body with gold-tantalum layerspresent only on the selected areas to a temperature of at least 850 C.but not exceeding about 1,700 C. to alloy the gold with the tantalum andto the silicon carbide with which said layers are in direct contact.

2. The process of claim 1 wherein the heat treating of the siliconcarbide body with gold-tantalum layers present only on the selectedareas is accomplished at a irst elevated temperature of from 850 to1,050 C. for a period of about 10 minutes and then heat treating thebody at a second elevated temperature of from l,500 to 1,700 C. for aperiod not exceeding about 15 seconds at the maximum temperature toalloy the tantalum, the gold, and the silicon carbide. 3. The process ofclaim 2 wherein the etchant is a saturated solution of sodium hydroxide.4. The process of claim 3 including the process step prior to etchingthe silicon carbide body of heat treating the silicon carbide body at anelevated temperature of from 280 to 320 C. for a period of time of up to16 hours. 5. The process of claim 4 wherein the heat treating isperformed in a vacuum.

6. The process of claim 5 wherein the heat treating is carried out in acontrolled atmosphere of a gas inert to the body.

7. The process of claim 2 wherein at least two layers of tantalum andtwo layers of gold are deposited on the selected areas of the siliconcarbide body; the total thickness of the layers of tantalum and gold isfrom 1,000 to 10,000 A, the volume ratio of tantalum to gold is fromabout 1:5 to

1:15, and the thickness of the layer of aluminum is 10,000 A.

2. The process of claim 1 wherein the heat treating of the siliconcarbide body with gold-tantalum layers present only on the selectedareas is accomplished at a first elevated temperature of from 850* to1,050* C. for a period of about 10 minutes and then heat treating thebody at a second elevated temperature of from 1,500* to 1,700* C. for aperiod not exceeding about 15 seconds at the maximum temperature toalloy the tantalum, the gold, and the silicon carbide.
 3. The process ofclaim 2 wherein the etchant is a saturated solution of sodium hydroxide.4. The process of claim 3 including the process step prior to etchingthe silicon carbide body of heat treating the silicon carbide body at anelevated temperature of from 280* to 320* C. for a period of time of upto 16 hours.
 5. The process of claim 4 wherein the heat treating isperformed in a vacuum.
 6. The process of claim 5 wherein the heattreating is carried out in a controlled atmosphere of a gas inert to thebody.
 7. The process of claim 2 wherein at least two layers of tantalumand two layers of gold are deposited on the selected areas of thesilicon carbide body; the total thickness of the layers of tantalum andgold is from 1,000 to 10,000 A, the volume ratio of tantalum to gold isfrom about 1:5 to 1:15, and the thickness of the layer of aluminum is10,000 A.